As used herein, a "semiconductor device" is a silicon chip (die) containing circuit elements on a face thereof, and a "semiconductor device assembly" is a silicon chip contained within a package and connected (wired or bonded) to conductive leads which exit the package.
The patent literature is replete with examples of semiconductor device assemblies having a plurality of leads exiting the package. For example:
Commonly-owned U.S. Pat. Nos. 4,771,330 and 4,880,419 disclose an integrated circuit chip 10 mounted to a die attach pad 22. Lead fingers 16 radiate outward from the area of the die attach pad. Bond wires 18 connect the die to the inner ends of the lead fingers 16. The outer ends of the lead fingers 16 are bonded (26) to package leads 28 which extend outward from the enclosing package 32. These two patents are examples of epoxy encapsulated semiconductor device assemblies.
Ceramic packaged semiconductor device assemblies are also common, examples of which are found in U.S. Pat. No. 4,899,207 and U.S. Pat. No. 4,903,113.
The complexity of modern semiconductor devices results in a high number of leads. Lead counts in excess of one hundred are not uncommon. For high lead count packages, the leads are necessarily very thin and closely spaced. Spacing on the order of 15-25 mils (center-to-center) is not uncommon. Evidently, these leads are very delicate, and extreme care must be exercised in handling the package to avoid damaging the leads so that they can be properly aligned with conductors on a printed wiring board (e.g.) to which the packaged device is ultimately mounted. Common damage modes include: 1) physical distortion of the leads, or 2) removal of plating from the leads due to scraping (the leads are commonly gold-plated).
In the process of packaging a semiconductor device, it is typical that the semiconductor die is inserted into an opening in the package. The package already has external leads exiting the package body, and lead fingers within the package body. The lead fingers are bonded (or wired) to the external leads. After the semiconductor device is connected to the lead fingers, a lid is applied over the package opening to seal the device within the package. During these packaging steps, there are many opportunities for causing damage to the external leads while handling and processing the package.
FIG. 1 shows a typical semiconductor device assembly 100, for which the present invention is especially pertinent. The assembly includes a square, flat ceramic package 102 containing a semiconductor die (not shown) within a cavity that is covered by a lid 104. A plurality of external leads 106 emanate from each side of the package 102, generally coplanar with the bottom surface of the package. Only eight leads per side are shown, for illustrative clarity. During installation of the semiconductor device in the package, the outer ends of the leads 106 are typically connected together by a lead frame ring 108 which helps support the leads. The leads 106 and the ring 108 are formed of a single patterned piece of metal (e.g., gold-plated copper) which is on the order of only about 8 mils thick. The leads themselves are usually about 350 mils long and 8 mils wide, and lead spacing of 15-25 mils is common. Evidently, the leads are extremely delicate.
After the semiconductor device is mounted and connected within the sealed package, the lead frame is excised. Often, there are holes 110 disposed at the four corners of the lead frame for locating the entire assembly in a machine that excises the lead frame.
As mentioned above, in the process of mounting the die in the package, and capping the package, the entire assembly is typically passed through an oven. Typically the lid is provided with a peripheral solder seal that will fuse with a corresponding metallic ring around the periphery of the package opening when exposed to a thermal cycle in the oven. Conveyor transports are common, and the packages may be placed on "boats" for the journey through the oven.
These "boats" are rigid structures having openings for the package body. These openings provide access from the bottom of the boat for automated machinery to lift the semiconductor assembly up from the boat during some of the packaging processes. Typically, the package is laid down directly on the boat, with the leads supporting the entire semiconductor device assembly. Because the boat is the direct support for the semiconductor device assembly, contact between the leads and the boat occurs. During the installation and removal of the package from the boat, physical bending of the leads may occur because of the close tolerances necessary to maintain package alignment. At the points of contact, motion of the semiconductor device assembly on the boat has been observed to cause the lead coating (plating) to be worn or scratched away, exposing either the under coat or the base metal. Further, with existing boat transports, the ceramic package body is not directly supported, since the package is held in place by the lead structure. This has been observed to allow damage to the ceramic through chipping or cracking. Further, with existing boat transport techniques, the leads are exposed to the operating environment during the above-mentioned processes, and contamination of the leads has been observed to occur from various forms of splatter.